Glucose is an optically active substance. Its concentration in a given solution can be quantitatively measured using polarimetric techniques. The rotation of plane polarized light by an optically active substance is well known. The concentration of an optically active substance is related to the magnitude of the rotation observed, .theta., the wavelength of the light, .lambda., the material's thickness, D, specific rotation, [.alpha.], and its temperature, t. ##EQU1##
If plane polarized light of incident intensity, I.sub.1, is passed through a polarizing material (analyzer) having its transmission axis at some angle, .alpha., to the plane of the light's polarization, the transmitted intensity, I.sub.2, is related to the square of the angle's cosine by Malus' law. EQU I.sub.2 =I.sub.1 Cos.sup.2 (.alpha.) (2)
Where .alpha. is the observed angle of rotation.
Human blood normally has a glucose concentration of between 80 and 120 mg/dl. The amount of polarization vector rotation imparted to plane polarized light with a wavelength of 670 nm by 100 mg of glucose dissolved in 1 dl of solution having a thickness of 1 cm and a temperature of 98.6.degree. F. is on the order of 0.004.degree.. Resolving rotation angles this small has become commonplace. However, in the presence of large noise, like that encountered in live human tissue, resolving angles of this magnitude becomes a formidable problem.
With an analyzer's transmission axis placed such that its parallel to the polarization axis of a beam of plane polarized monochromatic light which has a transparent optically active solution in its path, the intensity of light through the analyzer can be used to determine solution concentration, C. Combining equations 1 and 2: ##EQU2##
Equation (3) is completely amplitude dependant and assumes that there is no absorption, scattering or other amplitude related noise. The device described herein employs the use of a phase shifting technique which is amplitude independent, eliminating the largest source of noise. Additionally, the unique optics arrangement facilitates a technique for linearly magnifying this phase shift, providing improved instrument sensitivity. I also employ the use of sophisticated electronics capable of recovering a very small signal from large noise.